210 Variations of total dose to tumoral bed in breast irradiation

Purpose To evaluate the variations of the total dose given to tumoral areas in breast irradiation. Materials and Methods 50 patients consecutively submitted to CT for treatment planning were admitted in this study. The planes were elaborated on Varian Cadplan 2.61 both for whole breast irradiation (2 or 3 tangential fields of a cobalt unit, with compensator wedges, 2 Gy per fraction to 50 Gy) and for boost (fixed electrons field of adequate energy, 2 Gy per fraction to 10 Gy) according to ICRU 50 report. The doses were calculated localizing the tumoral areas with presurgical mammography or breast echography, description of the operation, possible presence of clips, and treatment planning CT. Results The dose on tumoral areas varies from a minimum of 46.55 Gy to a maximum of 53.45 (average 50.75, range 6.9  =  13%) with the tangential fields. The RBE varies from 55.25 to 64.9 Gy (average 61.05, range 9.65 = 16%). The total dose to tumoral beds varies from 57.8 to 63.5 Gy and the correspondent RBE from 69.1 to 76.9. Condusions The evaluation of the radiotherapy efficacy in breast irradiation must be also related to the dose to tumoral bed and not only to the prescribed dose. In facts, its high variations (and, consequently, its remarkable differences in RBE) might falsify the data interpretation, especially when the relationship between local relapse and boost usefulness is considered.